1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a magnetoresistive element having a structure in which a current is supplied perpendicularly to the plane of the element.
2. Description of the Related Art
The performance of magnetic devices, particularly magnetic heads, has been drastically improved by the discovery of the giant magnetoresistive effect (GMR). Specifically, application of a spin-valve film (SV film) to magnetic heads and magnetic random access memories (MRAMs) has brought about marked technical improvement in the field of magnetic devices.
The “spin-valve film” is a stacked film having a structure in which a nonmagnetic metal spacer layer is sandwiched between two ferromagnetic layers. In the spin-valve film, the magnetization of one ferromagnetic layer (referred to as a “pinned layer” or “magnetization pinned layer”) is pinned by an antiferromagnetic layer or the like, whereas the magnetization of the other ferromagnetic layer (referred to as a “free layer” or “magnetization free layer”) is made rotatable in accordance with an external field (for example, a media field). In the spin-valve film, a giant magnetoresistace change can be produced by a change of the relative angle between the magnetization directions of the pinned layer and the free layer.
Conventional spin-valve films are current-in-plane (CIP)-GMR elements in which a sense current is supplied parallel to the plane of the element. In recent years, much attention has been paid to current-perpendicular-to-plane (CPP)-GMR elements in which a sense current is supplied substantially perpendicular to the plane of the element because the CPP-GMR elements exhibit a greater GMR effect than the CIP-GMR elements.
When such a magnetoresistive element is applied to a magnetic head, a higher element resistance poses problems in regard to shot noise and high frequency response. It is appropriate to evaluate the element resistance in terms of RA (a product of the resistance and the area). Specifically, RA must be several hundred Ωμm2 to 1 Ωμm2 at a recording density of 200 Gbpsi (gigabits per square inch) and less than 500 Ωμm2 at a recording density of 500 Gbpsi.
In connection with these requirements, the CPP element has a potential to provide a high MR ratio even though it exhibits a low resistance on a trend of increasingly reducing the size of the magnetic device. Under the circumstances, the CPP element and the magnetic head using the same are expected to be promising candidates to achieve a recording density of 200 Gbpsi to 1 Tbpsi (terabits per square inch).
However, a metal CPP element in which the pinned layer, the spacer layer and the free layer (this three-layer structure is referred to as a spin-dependent scattering unit) are made of metal exhibits only a low resistance change rate. Accordingly, the metal CCP element is insufficient to sense very weak fields resulting from an increased density and is thus hard to put to practical use.
To solve this problem, a CPP element has been proposed which uses, as a nonmagnetic spacer layer, a nano-oxide layer (NOL) containing current paths extending across the thickness of the element (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-208744). Such a CPP element can increase both the element resistance and the MR ratio due to a current-confined-path (CCP) effect. Such an element is referred to as a CCP-CPP element hereinafter. Incidentally, a method for forming a layer mainly composed of an oxide in a magnetoresistive element has already been proposed (see Jpn. Pat. Appln. KOKAI Publication No. 2002-76473).
Compared to the metal CPP element, the CCP-CPP element has the following improvement effect. A metal CPP element was produced which had the structure of substrate/Ta [5 nm]/Ru [2 nm]/PtMn [15 nm]/Co90Fe10 [4 nm]/Ru [0.9 nm]/Co90Fe10 [4 nm]/Cu [5 nm]/Co90Fe10 [1 nm]/Ni81Fe19 [3 nm]/Cu [1 nm]/Ta cap layer. Ordering heat treatment for pinning the pinned layer by PtMn was carried out in a magnetic field at 270° C. for 10 hours. On the other hand, a CCP-CPP element having, as a spacer layer, a NOL formed by naturally oxidizing Al90Cu10 [0.7 nm], instead of the Cu spacer layer in the metal CPP element, was produced. The area resistances RA, the changes of the area resistance ΔRA, and MR ratios of these elements are shown below.
metal CPPCCP-CPPRA 100 mΩμm2 370 mΩμm2ΔRA 0.5 mΩμm2 5.6 mΩμm2MR ratio0.5%1.5%
As described above, the CCP-CPP element exhibits an improved MR ratio and an improved RA and thus has ΔRA one order of magnitude higher than the metal CPP element.
However, in spite of their good characteristics shown above, the CCP-CPP element is supposed insufficient to sense very weak field signals from a media with a high recording density of 200 to 500 Gbpsi. A trial calculation indicates that the MR ratio must be at least 3% at, for example, a recording density of 200 Gbpsi and RA of 500 mΩμm2. In order to obtain a sufficient signal-to-noise ratio, it is necessary to provide an MR ratio of at least 7%, that is, at least double the trial calculation. In view of these indices, the above value of the MR ratio is about half the required specification. Thus, it is difficult to put these elements to practical use.